Since the 1800's fingerprint information has been collected from human fingers and hands by means of ink and paper. For the purposes of this document, the term fingerprint is used to mean the skin surface friction ridge detail of a single fingerprint, partial fingerprint or any portion of the skin surface friction ridge of up to and including the entire hand. In recent years various electronic fingerprint scanning systems have been developed utilizing optical, capacitance, direct pressure, thermal and acoustic methods. Methods based upon acoustics or ultrasound have proven to be the most accurate, as they are virtually immune to the effects of grease, dirt, paint, ink and other image contaminants.
The ultrasound method employs a piezoelectric transducer that sends a longitudinal wave or pulse through an acoustic transmitting media. The pulse then partially reflects back at each media interface. The pulse reflected back to the sensing element may be used to measure the distance traveled by the pulse going and returning for each reflecting material interface. A time interval is monitored, so that valid signals can be identified using a process called range gating (biasing). Valid signals may be those which carry information that is desired. If the range of time during which a valid signal is expected to arrive at the sensing element is known, the control circuitry can be made to accept information produced by the sensing element during that time. Signals from each of the sensing elements are then processed, for example by converting each signal to a digital value representing the signal strength. Graphically displaying this information creates a three-dimensional contour map of the object (human finger or skin surface) that is in contact with the platen surface, with the depth of any gap structure (fingerprint valleys) detail being displayed as a gray-scale bitmap image.
Often, the device which generates the ultrasound energy is also used to detect the reflected ultrasound energy. It has been found that using the same device to generate and detect creates difficulties which often reduce the clarity of the image that can be obtained.